Commit some minor usability changes.

Adjust pulley radius to provide a full range of motion

Reset trim when average power is zero (avoids having to
spin up against extra resistance)

Set minimum resistance when under 10kph (previously 5kph)
Initial support for speed and power data from trainer

Extend the outbound packet size to 16 bytes, adding fields for
realtime speed and power data. Currently just copied from the
inbound data stream, but paves the way for future enhancements.